Abstract
Broadband dielectric spectroscopy was used to identify sub-Tg macromolecular motions for undoped and phosphoric acid - doped polybenzimidazole (PBI) films. Loss permittivity vs. frequency spectra showed three secondary relaxation processes labeled beta, gamma and delta in order of decreasing temperature for undoped PBI. As acid content in the membrane increased, peak broadening was observed for all relaxations while the peak positions shift closer to each other. The loss and storage permittivity vs. frequency curves for PBI membranes are displaced upwards with increased acid content which reflects increasingly greater polarizability due to increasingly greater populations of polar phosphoric acid groups. The high storage permittivity values observed for acid doped samples in the low frequency regime could be due to membrane | electrode interfacial polarization relaxation. The Kramers-Krönig integral transform was used to calculate pure loss permittivity relaxation peaks from experimental dc-free storage permittivity curves to eliminate obscuring dc conductivity contribution.
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